At one time, one of the principal devices for heating domestic hot water was a water jacket built into a wood or coal burning kitchen stove. Typically, a water storage tank would be positioned beside the stove, and water would be heated by thermal cycling from the water jacket to a tank. Such combinations were not completely safe as pressure buildup due to steam could result in an explosion. They have now been largely replaced by separate stove and water heating units, each being powered by its own heat source, typically gas or electricity. Currently manufactured hot water heaters typically consist of a tank which is fed from a cold water line and a thermostatically controlled heat source which generally maintains water temperature at a selected level in the range of approximately 120.degree. F. to 190.degree. F. In order to prevent an explosion in the event that a heating unit does not properly turn off, these hot water heaters are equipped with a combination pressure-temperature (PT) relief valve which opens a drain when one of the parameters exceeds a selected level, typically 150 PSI in pressure or 210.degree. F. Since the thermostatic controls of the heaters are quite reliable, typically having a life expectancy of approximately 10 years, the pressure-temperature sensitive relief valves are designed for only emergency operation and not repetitive operation. This is significant, as will be explained.
Fairly recently, and in the wake of a tremendous upsurge in the usage of wood burning space heaters, a number of water heaters have appeared for these space heaters. Typically, they include some form of heat exchanger which either directly receives heat from a combustion chamber of the space heater, or from air heated by the combustion chamber. Most frequently, the output of these hot water heaters is used as a supplementary heat source for heating water in a tank of a standard gas or electrically powered hot water heater of the type described above. Unfortunately, insofar as is know, none of the water heaters for space heaters provides means of prevention of their raising the temperature above the pop-off temperature of the PT valve of the basic hot water heater with which they are to be used. Of course, when water overheating does occur, the PT valve will open and hot water will be drained each time that it occurs. Not only does this provide a waste of water, but some, perhaps many, PT valve drains are not connected to the outside of a dwelling, and there may result in drainage on the floor of a dwelling. More importantly perhaps, and as described above, PT valves are simply not constructed for repetitive action, and thus they mail fail in such usage, and this may lead to an explosion.
Accordingly, it is an object of this invention to provide a space heater type hot water heater which overcomes the problems discussed and which is safe, convenient, and efficienct.